Properties such as protection from oxygen and moisture are of great importance in plastic packaging applications, as are toughness and processability. Generally, no single polymer is capable of providing the correct balance of all the properties that are required for various types of packaging applications. Polyolefins, for instance, are characterized by toughness, excellent moisture and water vapor resistance and fair chemical resistance, but suffer from poor gas barrier properties and variable processability. High nitrile polymers, on the other hand, have outstanding gas barrier properties but only modest water and vapor properties.
While it would be highly advantageous to incorporate the desirable characteristics of each type of polymer into a single material, barrier resins such as high nitrile polymers are not readily combined with polyolefins. Heretofore, when it has been desirable to form a multilayered film product comprising outer layers of tough, moisture resistant polyolefins and an inner layer providing gas barrier protection, it has been necessary to employ tie layers therebetween because of the lack of adhesion and incompatibility of the polyolefin and barrier resin layers. Thus, where three layers are functional, an additional two layers have been required to form such products.
U.S. Pat. No. 4,339,502, for instance, is directed toward a laminated structure comprising nitrile barrier resin and thermoplastic resin layers joined by a tie layer comprising a nitrile barrier resin and a vinyl aromatic diene block copolymer rubber.
U.S. Pat. No. 4,359,506 is directed toward a laminated structure which combines outer layers of a nitrile barrier resin and a thermoplastic resin with a tie layer comprising a mixture of a nitrile barrier resin, a halogenated polyolefin and a thermoplastic resin.
U.S. Pat. No. 4,415,520 is directed toward a process for bonding together a nitrile barrier resin with a thermoplastic resin to produce a laminate. A tie layer comprising a mixture of a nitrile barrier resin and a halogenated polyolefin and a thermoplastic resin other than the first two polymers is disclosed for bonding the first two film layers together.
The multilayer concept is also employed to obtain barrier structures containing poly(ethylene-vinyl alcohol) as a barrier layer bonded to polyolefins via adhesive tie layers. For a discussion of this work, see Foster, R. "EVAL RESINS, THE SUPER CHALLENGE"; proceedings of Food Plast. 84/85, March, 1985.
It has now been found possible to structure the two polymers into a multilayered material by the use of a heterogeneous inner layer. In particular, an inner layer is formed comprising a barrier polymer, or polymer providing another desired property, and a polymer compatible with the exterior layer. The inner layer provides a dual function of gas barrier protection, or other property, and adhesion promotion to the exterior layers.